The invention relates generally to second harmonic generation and frequency mixing, and, more particularly to an efficient non-linear optical coupling system capable of second harmonic generation and frequency mixing wthout deleterious reflection losses.
Great need has arisen in areas of IR radar and induced chemical reactions, for example, to efficiently convert infrared wavelengths to those values necessary for effective operation of these systems. Since lasers generally have fixed or discrete frequencies there are many wavelengths regions where there are no acceptable laser sources available. To overcome this problem a variety of techniques have been utilized.
The most common technique is to utilize the optical non-linearity exhibited by non-centrosymmetric crystals for second harmonic generation, frequency mixing and optical parametric generation. For efficient conversion the generated polarization and electromagnetic wave must propagate in phase, but due to dispersion, radiation at different frequencies will normally propagate at different phase velocities. The most commonly used technique to overcome this problem exploits the double refraction exhibited by certain crystals. Certain asymmetric crystals such as calcite are doubly refracting because in them light can travel at two different velocities, described as ordinary and extraordinary. These velocities actually vary with propagation direction and polarization as well as with wavelength. For instance, in potassium dihydrogen phosphate (KDP), a piezoelectric crystal commonly used for harmonic generation ordinary fundamental light at 6,934 angstroms at an angle of 50.degree. to the optic axis of the crystal travels at exactly the same velocity as extraordinary harmonic light at 3,471.5 angstroms. When this direction is used for harmonic generation, the retardation of the ultraviolet harmonic light due to dispersion is precisely compensated by the higher velocity of extraordinary light at the harmonic wavelength. This technique has made possible an increase in coherence length from a thousandth of a centimeter to more than a centimeter.
Another technique of avoiding the interference problem is applicable in ferroelectric crystals such as barium titanate. Ferroelectric crystals can be obtained in the form of a multilayered sandwich in which the layers are regions, called domains, that have different properties. In barium titanate adjoining domains are completely equivalent except that one is inverted with respect to the other. The phase of the harmonics generated in successive domains is reversed, with the result that the interference effect is particularly offset and harmonic generation greatly enhanced.
The systems of the past, however, still leave much to be desired since these processes of generating new wavelengths utilize birefringent crystals. Unfortunately crystals which have the highest non-linearity and thus have the potential for producing the highest output power are non birefringent. In addition, in the 5-10 .mu. region no acceptable crystals are available, in that they are either difficult to make or are of poor optical quality.
The problem therefore arises on how to effectively utilize non-birefringent crystals which are more easily manufactured and are generally of good optical quality in second harmonic generation and frequency mixing.